Compared with the single principle oil-water separation method, the combined principle of swirl + membrane is a new oil-water separation method. In order to improve the performance of downhole oil-water separation, a combined principle oil-water separator is discussed. The physical and mathematical model of swirl + membrane combined oil-water separator is established. The flow field distribution is calculated by the numerical simulation method. The effects of different split ratios, inlet flow velocities and inlet oil volume fractions on the performance are systematically studied. The results show that: the change of split ratio affects the flow rule of the first stage and the second stage oil outlet, and performance accounting should be carried out to ensure the effect of two-stage separation; with the increase of inlet flow rate, the performance of swirl + membrane separation becomes better, and the performance of swirl + membrane separation is poor when the flow rate is too low; with the increase of inlet oil volume fraction, the proportion of oil phase separated by swirl decreases, and the proportion of oil phase left for membrane separation increases, that is, when the oil volume fraction is high, the application of swirl + membrane combined oil-water separation is more necessary.